Significant research efforts have been devoted toward the development of water-based emulsion coatings as replacements for the solvent-based coating formulations for use in industrial and architectural applications. However, water-based coatings have found somewhat limited acceptance in wood finishing operations. The use of water-based coatings is generally known to those in the coatings industry to have at least two disadvantages over solvent-based coatings: water-soluble chromophoric compounds, such as tannin, on and in the wood surface bleed through the applied water-based coatings to stain, discolor and detract from the resulting finish quality; and, when applied under and exposed to certain environmental conditions, physical properties such as scrub resistance, water whitening resistance, corrosion resistance, hot block resistance; and fire, smoke, nicotine, pen and ink stain blocking and adhesion of the water-based coating to the wood substrate can be a problem.
In an attempt to improve the tannin stain blocking of water-based coatings, reactive pigments have been incorporated into the coating. Although reactive pigments are generally effective tannin stain blocking agents, they, however, have limitations. Reactive pigments can cause stability problems, such as viscosity increase and polymer gelation, and are known to be environmentally unfriendly.
What is required is a method for blocking tannin stains in coatings applied to tannin-containing substrates, such as, for example, wood, cardboard, paper, wood compositon particle board, Masonite, and the like, that is effective without the use of either volatile organic solvents or reactive pigments.
What has been found is a novel method for employing an organosilane-modified composition for the purpose of improving tannin stain blocking in coating compositions over tannin-containing substrates. There are several advantages of the method of the invention:
(1) It reduces the leaching of water-soluble chromophoric compounds, on or in the wood or other tannin-containing substrate surface, especially the highly concentrated tannins found in wood knots, from bleeding through a subsequently applied water-based finish; PA1 (2) It provides a water-based wood finishing composition and process that is equal in tannin stain blocking performance to solvent-based and reactive pigment containing systems. PA1 (3) It provides a water-based unpigmented coating which is generally, coalescent-free, solvent-free and contains low levels of volatile organic compounds (VOCs) when compared to solvent based coatings. PA1 (1) an acid-functional polymer formed from about 0.1% by weight to about 20% by weight, based on the total weight of said polymer, of at least one acid-functional monomer; and PA1 (2) 0.1% to about 20% by weight, based on the weight of the polymer, of an organosilane, wherein the acid-functional polymer has an acid level from about 0.1% to about 20% and wherein the organosilane has a molecular weight of from about 100 to about 10,000. PA1 (1) an acid-functional polymer formed from about 0.1% by weight to about 20% by weight, based on the total weight of said polymer, of at least one acid-functional monomer; and PA1 (2) 0.1% to about 20% by weight, based on the weight of the polymer, of an organosilane, wherein the acid-functional polymer has an acid level from about 0.1% to about 20% and wherein the organosilane has a molecular weight of from about 100 to about 10,000. PA1 (1) Qualitatively on a scale of 1 to 10 by assigning the coating a rating of 10 (100% tannin blocking) to a rating of 1 (10% tannin blocking). In this method of judging tannin stain blocking, 2.0 units are considered significantly different. PA1 (2) Spectrophotometrically by readings taken with a Minolta Data Processor DP-100. Less-positive `b` values of an Lab reading indicate better tannin stain blocking performance, than more positive `b` values. In this method of judging tannin stain blocking, 0.4 units are considered significantly different. PA1 (3) Comparatively by comparing the measured `b` values of an experimental primer that is topcoated with the `b` value of the fail control. This is particularly useful when the staining substrate is very rich in tannins and other chromophoric material. In this method, the formula for percent tannin stain blocking is listed below. EQU % Stain Blocking=10+[b.sub.fail -b.sub.experimental /(b.sub.fail /90)] PA1 b.sub.fail =`b` value of the fail control PA1 b.sub.experimental =`b` value of the topcoated experimental primer.
This improvement in tannin stain blocking produced by the method of the invention is accompanied by unexpected improvements in physical properties, including reduced viscosity; chalk, alkyd and weathered wood adhesion; scrub resistance, water whitening resistance, corrosion resistance, hot block resistance; and water, fire, smoke, nicotine, pen and ink stain blocking.
The organosilane-modified coating useful in the method of this invention may be substituted for water-based and solvent-based coatings, as well as coatings containing reactive pigments such as, for example, zinc oxide and barium metaborate.